Shadows through a solid object

I think that if you know your shadow-receivers you can discard other objects in your shadow drawing, or draw other objects with writing 0 to stencil buffer

Hmm, that almost sense except that clipping the shadow volume by all the other receivers is quite expensive (the receivers can be very complex geometry). I suppose the trick would be to resolve the other receivers into simple clipping planes and snaffle the volume that way.... but that's also far too expensive surely?

[edit: receivers, not casters]

The bridge needs to cast a shadow too. If you don't want the bridge to cast a shadow, then perhaps you could cap the shadow volume somewhere between the bridge and the river.